1. Field of the Invention
This invention relates to a degradation discrimination system of an internal combustion engine exhaust gas purification system, more particularly to a system for discriminating whether an exhaust gas purification system comprising an adsorbent for adsorbing unburned components including the hydrocarbons (HC) in the exhaust gas generated by an internal combustion engine, has degraded or deteriorated.
2. Description of the Related Art
Internal combustion engines are ordinarily provided with a catalyst (a three-way catalytic converter) in the exhaust system which removes HC, NOx and CO components in the exhaust gas generated by the engine. However, when the catalyst is not activated, for example, at the time of engine cold-starting, unburned components of the exhaust gas including unburned HC are released immediately into the atmosphere.
For that reason, there has been proposed an exhaust gas purification system which has an adsorbent made of a zeolite material or some similar material installed in a bypass exhaust gas passage branched from the exhaust pipe at a location downstream of the catalyst, which merges into the exhaust pipe at a downstream point and has a switch-over valve which opens or closes the bypass exhaust gas passage. The switch-over valve opens the bypass exhaust gas passage when the engine is started to introduce the exhaust gas such that the adsorbent adsorbs unburned components including the HC when the catalyst is not activated and closes the bypass exhaust gas passage such that the adsorbent desorbs the adsorbed component and the desorbed components are thereafter recirculated at a position upstream of the catalyst after the catalyst has been activated.
Since a desired exhaust gas purification can not be achieved if any degradation or abnormality arises in such an engine exhaust gas purification system, Japanese Laid-Open Patent Application No. Hei 8 (1996)-93,458, for example, proposes the technique to discriminate whether any degradation or abnormality arises in the system. Another Japanese Laid-Open Patent Application, No. Hei 8 (1996)-218, 850 proposes a similar technique.
Specifically, Japanese Laid-Open Patent Application No. Hei 8 (1996)-93,458 proposes providing an HC sensor at a position downstream of the bypass exhaust gas passage for detecting the HC concentration of the exhaust gas at that location. In this prior art, the HC sensor detects the HC concentration in the adsorption mode and in the desorption mode respectively. The detected values are compared with predetermined values and based on the result of the comparison, it is discriminated whether any trouble has occurred in a mechanical part such as a switch-over valve.
In addition, this prior art proposes providing the HC sensor at a recirculation passage to determine the total amount of HC being recirculated, and by comparing the determined amount with a predetermined value, it discriminates whether a trouble has occurred in a mechanical part such as a valve for opening/closing the recirculation passage.
Japanese Laid-Open Patent Application No. Hei 8 (1996)-218,850 proposes providing O2 sensors at a position upstream of the adsorbent, but downstream of the catalyst and at a position upstream of the recirculation passage. In this prior art, a time lapse until the rich/lean signals of both the sensors become equal to each other is measured and based on the measured time, it discriminates whether any abnormality arises in the system such as at the adsorbent.
Specifically, based on the assumption that the air/fuel ratio in the recirculation passage becomes richer than that at a position downstream of the adsorbent since the desorbed HC is recirculated together with the exhaust gas, but becomes equal to the air/fuel downstream of the adsorbent after the desorbed HC has been purified, the second prior art conducts the discrimination by measuring the time lapse until the outputs of the air/fuel ratio sensors coincide. Similarly, the second prior art proposes providing an HC sensor, instead of the oxygen sensor, to discriminate the occupance of abnormality based on the HC concentration in the desorption mode.
Although the first prior art mentioned in Japanese Laid-Open Patent Application No. Hei 8 (1996)-93, 458 monitors the behavior of HC directly through the HC sensor, the first prior art can not discriminate whether the abnormality has occurred in the adsorbent or any other part such as switch-over valve. In other words, the first prior art can not discriminate the abnormality or degradation arising in the adsorbent accurately.
The second prior art mentioned in Japanese Laid-Open Patent Application No. Hei 8 (1996)-218,850 can similarly discriminate the abnormality or degradation arising in the system including the adsorbent by detecting the HC concentration in the desorbtion mode. However, since the second prior art does not detect the concentration or amount of HC flowing into and adhering to the adsorbent in the adsorption mode, the second prior art is not always satisfactory in the discrimination accuracy if there are variances in the amount of HC. The same argument will also be applied in a case when the amount of recirculation fluctuates.
An object of the invention is therefore to provide a degradation discrimination system of an internal combustion engine exhaust gas purification system having an adsorbent installed at a bypass exhaust gas passage branched from the exhaust pipe that is opened by a switch-over valve at engine starting to introduce the exhaust gas such that the adsorbent adsorbs unburned HC components in the exhaust gas generated by the engine and is closed such that the adsorbent desorbs the HC component and the desorbed HC component is thereafter recirculated at a location upstream of a catalyst, which enables to discriminate the degradation or deterioration of the adsorbent accurately by directly monitoring the behavior of the HC component including the adsorption mode during which the adsorbent adsorbs the HC component through an HC concentration detecting means.
In order to achieve the object, there is provided a system for discriminating degradation of an exhaust purification system of an internal combustion engine having an adsorbent installed at a bypass exhaust gas passage branched from an exhaust pipe of the engine and merged into the exhaust pipe at a location downstream of the adsorbent, the bypass exhaust gas passage being opened by a switch-over valve at starting of the engine to introduce the exhaust gas such that the adsorbent adsorbs unburned HC in exhaust gas generated by the engine and being closed such that the adsorbent desorbs the adsorbed HC and the desorbed HC is thereafter recirculated at a position upstream of a catalyst, comprising: an HC sensor installed in the bypass exhaust gas passage at least one of a first position at the adsorbent and a second position downstream of the adsorbent for detecting concentration of HC introduced in the bypass exhaust gas passage in an adsorption mode where the adsorbent adsorbs HC; and adsorbent degradation discriminating means for comparing the detected concentration of HC with a threshold value and for discriminating whether the adsorbent has degraded based on a result of comparison.